Electrodes used in electric arc furnaces are usually made of carbon or graphite and comprise discrete sections joined together to form an electrode column. The joining of these electrode sections in electric arc furnaces is typically by means of threaded portions formed in the section ends. The threads on these end portions which combine to make a joint are made of the same material as the electrodes themselves. In many applications, it has become standard to use a tapered threaded joint for its superior strength.
The threaded joint between electrode sections may consist of a tapered threaded projection formed on one end of an electrode section and connected to a complimentarily tapered threaded socket formed in one end of another (male-female). It may also consist of a double ended tapered threaded nipple or connecting pin joining two electrode sections each having complimentarily tapered threaded sockets formed in an end. The taper angle has typically been 9.46.degree. in standard electrode connecting pin joint configurations. National Electrical Manufacturers Association (NEMA) Standards Publication no. CG 1-1982 discloses this taper for threaded electrode ends of various sizes. Higher taper angles are generally used for male-female joints, typically from approximately 16.degree. to 35.degree.. As used herein, "taper angle" refers to the included angle between a straight line running along the top surface of the threads and the longitudinal center line axis of the electrode.
If the thread profile of a threaded projection or socket is viewed in cross-section, it will be generally seen to have straight thread crests and flanks, and curved thread roots. As used herein, "cross-section" refers to a plane section through the electrode which includes the longitudinal centerline axis of the electrode. Since the commercial introduction of tapered threaded carbon-graphite electrode joints, the thread crest angle has been the same as the taper angle of the threaded projection or socket. As used herein, "thread crest angle" refers to the included angle between the thread crest cross-section and the longitudinal electrode axis. It appears that this configuration was selected because of the ease of fabrication of thread cutting tools and the limitations of previously existing machinery. Graphite electrode thread crests having an angle equal to the taper of the projection or socket are disclosed in the aforementioned NEMA publication no. CG 1-1982. The present invention relates to the unique problems associated with carbon-graphite electrode threaded joints and the selection of thread crest configurations which overcome these problems.
Typically, electrode columns made up of joined electrode sections will project through the roof of electric arc furnaces and into the furnace chamber where an arc will be struck. The portion of the electrode column within the furnace chamber can be a single electrode section or it can be a number of electrode sections joined by the threaded joints describe above. During operation of the furnace, the electrode column is consumed from the bottom end at which the arc emanates. Because it is necessary to maintain a controlled arc length, the electrode column must be fed down into the furnace to compensate for the electrode consumption. Adequate electrode column length is assured by adding new electrode sections to the top of the electrode column which protrudes from the furnace roof.
The joining of electrode sections in a mill environment is often performed without the aid of sophisticated devices. A mill operator will first suspend a fresh electrode from a crane and axially align the fresh electrode over the electrode column section protruding from the furnace roof. The operator will then longitudinally downwardly advance the fresh electrode toward the electrode column. In one method of joining, the operator will lower the fresh electrode without rotation until its threads make contact with the threads of the electrode column section. He will then commence to screw the two electrodes together. In another method of joining, the operator will commence rotating the fresh electrode about its longitudinal axis before the threads make contact to screw the electrodes together.
The operator normally rotates the fresh electrode manually with the aid of an electrode turning fixture, for example, a chain wrench. The operator may also utilize a threaded stem device between the crane and the fresh electrode which, when fitted with a screw thread that has the same pitch as that of the threaded joints, allows the operator to longitudinally advance and rotate the fresh electrode toward the electrode column at the exact rate of advancement of the electrode threads.
Even with the aid of a crane and turning devices, the joining of the threaded electrode section ends does not always proceed smoothly. The great size and weight of the electrode sections (up to 28 inches diameter and 4200 lbs weight for graphite electrodes, and 55 inches diameter and 15,000 lbs. weight for carbon electrodes) necessitate the use of large cranes or hoists which generally have imprecise controls and which therefore cannot locate the section with a great deal of precision. In addition, because tapered threads are used, the threaded projection of one electrode must be inserted deep into the threaded socket of the other electrode and out of easy view of the operator before the threads align and mate. The result of this practice is that the joining of the complimentarily threaded section ends is accomplished with much repositioning and inadvertent bumping and scraping between the threads at each end. This can lead to thread breakage.
The aforementioned problem of inadvertent bumping and scraping and possible breakage of threads can occur even if the two electrode sections to be joined are held in perfect axial alignment during joining. This may occur if the properly aligned fresh electrode section is advanced toward the electrode column with or without rotation of the fresh electrode section about its longitudinal axis. The thread crest of one electrode may not properly mate with the thread root of the other electrode and instead the thread crests of the two electrodes may become jammed, i.e., locked in wedged engagement with each other. It may also occur if the threaded stem is used and advancement and rotation are not begun with the proper longitudinal distance between complimentary thread points (the center of a thread crest on the projection and the center of a thread root in the socket, for example). If this distance is not equal to an integer multiple of the electrode thread pitch, thread crests of the projection and socket may become jammed.
At this point in the process, the operator is faced with the problem of freeing the jammed threads. Because of the difficulty of reversing a chain wrench (the usual electrode turning fixture) and the great force required, the operator will not normally attempt to unscrew the overhead fresh electrode. More commonly, the operator will use the crane to jog the upper fresh electrode up or down relative to the electrode column to unjam the thread crests. Once the threads are unjammed, the process is restarted and the fresh electrode is again moved in an attempt to align the thread crests and roots in proper longitudinal relationship. When the thread crests and roots of the two electrode sections are correctly aligned, they may be screwed together properly.
Since the threads (and the electrodes) are made of carbon or graphite which are relatively fragile materials as compared to metals, jogging the electrodes to free jammed threads can cause fragments of the threads to break off. If this occurs, one resulting problem is that the thread strength of the joint is weakened. An even greater problem arises if the thread fragments are trapped between the two threaded sections, preventing proper engagement of mating threads. This can easily occur when, as is normally the case, the fresh electrode section overhead has the threaded projection (either integral with the electrode section or as a threaded connecting pins screwed into a threaded socket) and the end of the electrode column below and protruding from the roof contains the threaded socket. Fragments can still be trapped if the positions of the thread projection and socket are reversed. The faulty connection will result in, among other things, an increase in electrical resistance which causes excess heating and thermal stress. Electrode column vibration during furnace operation may result in further problems by causing the trapped thread fragments to break into smaller pieces, thereby loosening the joint. The loose joint will be weak and susceptible to full unscrewing of the lower column section.